Motorized beds have been used for years as a convenience for hospital patients and others to position various portions of the bed into a configuration which provides maximum comfort. A motorized bed also makes it relatively easy for a user to change the position of the bed as frequently as desired.
One of the concerns in making a motorized bed is protecting a user from any chance of shock or . other hazard due to electrical leakage currents. Accordingly, safety standards have been established by Underwriters Laboratories which include testing the various motors and controls used in motorized beds for maximum leakage current under various conditions. One of these conditions is a reverse polarity connection. Typically, a motorized bed will have an electrically "hot" conductor which carries the A-C power to operate the motors, a neutral conductor, and a ground or equipment ground conductor. In the reverse polarity connection test, the hot and neutral conductors are reverse connected such that the electrically hot line from the incoming power is connected to the neutral of the motorized bed circuit and, similarly, the neutral conductor from the power supply is connected to the conductor which is ordinarily connected to the hot side of the power supply. In this reverse polarity connection, the UL 544 standard is for a maximum of 100 microamps of leakage current.
The motors typically used in previous motorized bed circuits were 16 slot stator motors which had no trouble meeting this reverse polarity leakage current standard. However, recently, some manufacturers have converted to a 24 slot stator motor. With the 24 slot motor there is not only a savings in cost, but also an increased motor torque available which permits its use in a greater number of bed designs and thereby serves to reduce inventory, and also results in a higher speed operation for some directions of bed movement. Unfortunately, with the increased number of stator slots, there is an increased leakage current induced in the reverse polarity condition such that when multiple motors are energized the UL 544 standard of 100 microamps is not met as up to 125 microamps of leakage current has been detected in some tests. While there have been some circuit modifications suggested to meet this standard, they are not particularly desirable. One suggested modification involves the use of a relay control which interlocks the motors such that only one motor is energized at any particular time. In a bed with multiple motions, this interlocking reduces the facility of movement and adjustability thereby rendering it more time-consuming and less convenient for a user to adjust the bed position. Still another approach has been to utilize relay contacts to disconnect the motor windings upon reverse polarity connection. However, this is also undesirable in that it inordinately increases the cost involved for the control.
To solve these and other problems, the inventors herein have succeeded in designing and developing a simple circuit for injecting an offset current under the reverse polarity connection conditions which is substantially 180.degree. out of phase from the leakage current such that it offsets a portion of the induced leakage current and reduces it to a value below the 100 microamp maximum. Typically, a control transformer is included in the control portion of a motorized bed as it is desired to operate the control circuit and hand control at a voltage much lower than the voltage used to operate the motors themselves. Taking advantage of this transformer, the inventors have utilized a transformer with an additional winding on the primary side, this auxiliary winding being wound for reverse polarity to the motor windings. The auxiliary transformer winding is then connected between neutral and ground with a series resistor such that as the neutral conductor is connected to the hot side of the power supply, an offset current is injected through the series resistor into the ground conductor. As this injected offset current is 180.degree. out of phase, it serves to cancel out and reduce the leakage current to a value below the 100 microamp standard.
Although this same transformer winding and series resistor serves to inject a leakage current under normal connections, which because of its polarity is in phase with the induced leakage current, its value of a nominal 30 microamps when added to the approximately 20 microamps of leakage current experienced under normal connections totals only 50 microamps, this 50 microamps of leakage being still well below the 100 microamps standard. Thus, the net effect of adding this auxiliary transformer winding and series resistor is to reduce the reverse polarity leakage current from approximately 125 to a value just below the 100 microamp standard of UL 544 without increasing the leakage current for the normal connection condition beyond the 100 microamp standard. As a result, the inventors have succeeded in making a circuit change of nominal cost which brings the whole circuit back within UL 544 standard when used with 24 slot stator motors without limiting the overall operation of the bed as in other suggested circuit modifications designed to meet the UL 544 standard.
While the principal advantages and features of the invention have been disclosed and described above, a fuller understanding thereof may be attained by referring to the drawing and description of the preferred embodiment which follow.